The present invention relates to receivers, utilized for range measurements, which use radio signals modulated with navigation message symbols, and which function in environments where signal power may not be sufficient for proper operation of conventional receivers.
Current navigation GPS/GLONASS receivers usually track satellite signal parameters with tracking estimators such as phase lock loops (PLLs) and delay lock loops (DLLs).
A typical global positioning satellite signal comprises a high frequency carrier signal which is modulated by one or more pseudo-random number sequences (PR-codes) of lower frequency, which in turn is modulated a 50 Hz data stream which provides navigational information. The task of a phase-lock loop is to track the high-frequency carrier signal or a down-converted version thereof, the latter being the typical situation, and the task of a DLL is to track a pseudo-random number sequence. The 50 Hz data stream in the received satellite signal is a barrier for the improvement of tracking system threshold characteristics. In this case, a symbol demodulation method determines the ultimate threshold parameters of the tracking system.
The same problem of interference immunity is the key performance for modern navigation receivers that employ processing of discrete signal records with high-speed signal processors instead of tracking measuring estimators.
In order to increase the interference immunity of the measurements of the signal carrier phase and code delay (pseudorange), it is necessary to eliminate the signal symbol modulation of the 50 Hz data stream. In this case, an extra receiver, located in certainly favorable conditions, is used to provide error free symbol demodulation. Let us call such a receiver a base station, while the other receiver whose location is to be measured will be called a rover station. One of the interference immunity methods for the receiver-rover system is to transmit the demodulated symbol stream from the base station to the rover station. (Lawrence R. Weill, xe2x80x9cUltra-High Performance Differential Systems: Extending Accuracy and Weak-Signal Tracking Capability,xe2x80x9d Proceedings of the 9th International Technical Meeting of the Satellite Division of the Institute of Navigation, ION GPS-96, Sep. 17-20, 1996, pages 1753-1762).
The drawbacks of this method are, first, the necessity of the base station to provide the rover station with navigation symbols; second, the fact that the method""s efficiency for noise immunity tracking depends upon the capabilities of a radio link (e.g., radio link delay, partial loss of symbols due to short-term interruptions in the radio link or fading); third, the necessity to account for the delay in the transmission of navigation symbols from the base station to the rover, which results in a substantial complicating of the rover""s firmware.
Another method (U.S. Pat. No. 6,052,081) suggests that a rover remove P/N data and create a record of the satellite data message, and thereafter transmit the record to a base station. The base station then performs a cross correlation of the record transmitted from the rover with a xe2x80x9creferencexe2x80x9d record made by the base station. From this cross correlation, the base station determines the time of the transmitted record instead of using the rover to demodulate this information (ephemeris reading). With such a processing, the key moment is the use of navigation symbols demodulated on the base station.
The disadvantages of the above method are, first, the usage of the auxiliary base station for determining the satellite time for the signal that was received at the rover; second, the method allows measurements only of pseudoranges at the rover, and not carrier phases of the received satellite signals.
Accordingly, there is a need to provide for a more simple and reliable way of improving the tracking system threshold characteristics for weak signals and the interference immunity for all signals in receivers of GPS and GLONASS satellite signals.
The present invention is directed to methods and apparatus for predicting navigation symbols for use in a global positioning tracking processes. The navigation symbols are transmitted from a global positioning satellite in the form of a carrier signal modulated by a data-stream signal comprising the symbols. The navigation data symbols are conveyed in a sequence of one or more repeating frames, with a first group of symbols being repeated within the stream once every number M of frames at predetermined locations in the frames. For the present invention, M can be equal to or greater than 1.
An exemplary method according to the present invention comprising the first step of detecting the symbols of the first group within the frames of the information data stream signal, and the second step of storing in a memory a sequence of the detected symbol values of the first group over at least M frames. The method thereafter provides the sequence stored in the memory to the global positioning tracking process. The exemplary method may subsequently monitor the data-stream signal to detect a change in the symbol values of the first group. This may be done by detecting the symbols of the first group within subsequently transmit frames, comparing their values with the sequence of previously detected symbol values stored in memory to find differences, and counting the number of differences encountered since the start of a groups of frames. If the number of differences exceeds a predetermined number, such as for example a number between 10 and 30, the method may then repeat the above described first and second steps.
Another exemplary method according to the present invention comprising the step of detecting the symbols of the first group within the frames of the information data stream signal, and the step of storing in a memory a sequence of the detected symbol values of the first group over at least M frames. The method thereafter modifies one or more correlation signals which are input to a discriminator of a satellite tracking channel (e.g., PLL discriminator, DLL discriminator) in relation to the sequence stored in the memory.
The use of navigation symbol prediction in the above ways enables a substantial improvement in the receiver""s tracking threshold performance (for example, up to 10 dB).
Accordingly, it is an object of the present invention to improve the tracking performance of satellite tracking channels for GPS and GLONASS navigation satellites.
It is another object of the present invention to improve the interference immunity for navigation receivers.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention, the accompanying drawings, and the appended claims.